This invention relates to a fuel supply control method for internal combustion engines equipped with catalytic means for purifying exhaust gases, and more particularly to a method of this kind which is adapted to prevent an abnormal increase in the catalyst bed temperature of the catalytic means when the engine is operating in a certain high speed operating region.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
Also, in an engine having a three-way catalyst or a like catalytic means arranged in its exhaust system, it is generally employed to control the air/fuel ratio of the mixture to a theoretical mixture ratio in a feedback manner responsive to the output of an exhaust gas concentration sensor which may be represented by an O.sub.2 sensor, arranged in the exhaust system of the engine, to obtain the best conversion efficiency of unburned hydrocarbons, carbon monoxide and nitrous oxides in the exhaust gases emitted from the engine.
However, this feedback control based upon the output of the exhaust gas sensor cannot be applied when the engine is operating in a particular operating condition where the air/fuel ratio of the mixture needs to be controlled to a value different from the theoretical mixture ratio.
For instance, when the engine is operating in a certain high speed region, if the engine is operated with the air/air fuel ratio of the mixture controlled to the theoretical mixture ratio or a value close thereto, the bed temperature of the three-way catalyst arranged in the exhaust system of the engine can abruptly increase above a maximum allowable temperature. The rate of such increase in the bed temperature of the three-way catalyst can become higher with an increase in the intake pipe absolute pressure PBA. That is, if an air/fuel mixture having a theoretical mixture ratio or a value close thereto is supplied to the engine while the engine is operating in the above certain high speed region, the efficiency of combustion within the engine cylinders will become higher to increase the heat generated per unit mass of the air/fuel mixture, thereby increasing the temperature of the exhaust gases flowing through the three-way catalyst. Also, the higher the temperature of the exhaust gases, the higher the reaction rate of the three-way catalyst with the exhaust gas ingredients, and the resultant increased reaction heat causes the bed temperature of the catalyst to rise. Further, as the quantity of exhaust gases per unit volume of the catalyst increases, the catalyst bed temperature increases. Thus, as the exhaust gases increase in both quantity and temperature or catalytic reaction rate, the caltalyst bed temperature abruptly increases. Therefore, when the engine is operating in a high speed region, especially with a high engine load wherein the exhaust gas quantity is large, the catalyst bed temperature can easily exceed a maximum allowable temperature.